Monitoring of physiological signals for medicine and research presents unique problems that include the acquisition of this information in vivo and the transmission of this information to instruments external to the body where it may be stored and analyzed. These physiological signals may include neural waveforms that result from the firing of individual or groups of neurons, electroencephalogram (EEG) signals from the spatial summation of neural signals, electromyogram (EMG) signals that result from the activation of muscle tissue, and electrocardiogram (EKG) signals that result from contractions of the heart. Other physiological variables that may be monitored, using a suitable sensor, include, for example, temperature, pressure or pH; glucose, CO2 or phosphate concentration; and rate of perfusion. Since many of these physiological signals cannot be determined by non-invasive methods, they must be acquired by the use of implanted sensors, transducers, or electrodes and provided to the external device via transdermal catheters or percutaneous connectors. These catheters and percutaneous connectors typically include physical connections such as wires or cables that relay power, information or both to and from the interior sensors. Catheters and percutaneous connectors, however, are easily damaged, can create a significant potential for infection, and also may be subject to marsupialization by epithelial downgrowth.
Typically, wireless systems used today to provide a wireless communication between the implanted in vivo electronics and sensors and the external device rely on radio frequency (RF) systems. RF systems have several disadvantages, however. For example, for many implant sites RF wireless systems can be too large or heavy. A RF system that is too heavy and too dense compared to the surrounding environment may be subject to non-uniform accelerations compared to the physiological structures surrounding it and physically move, damaging or destroying the physiological structures that were to be monitored. In addition, RF wireless systems are themselves subject to RF interference from telecommunication devices, microwave ovens, computers, cell phones and other common electronic devices.
Within the United States almost 11,000 people each year are victims of a spinal cord injury, of which a significant portion result in at least partial disability of the injured individuals. Half of those injured annually are between the ages of 16 and 30 years old. Currently, there are more than 190,000 people living with some form of paralysis caused by a spinal cord injury. Extensive research is being carried out to help these individuals by monitoring and analyzing neural signals from the regions of the brain associated with muscular control and movement.
Neurons are the fundamental information unit of the nervous system and generate action potentials in order to transfer information from one location to another and to activate muscles for movement. These action potentials consist of roughly three nA ionic currents that flow in the extra cellular spaces between the neurons. These small currents are emitted from localized areas within the neurons, which are on the order of a few microns in size. This current spreads into the volume of the resistive aqueous fluid in the extra cellular space, creating a voltage gradient or electric field therein. The information transfer that occurs in the nervous system can thus be tapped or accessed by implanting electrodes into the extra cellular spaces. These electrodes include small electrically active sites for transducing the small voltages that are created by the neurons and transferring them to instrumentation for analysis outside the body.
Research in the field of neuroprosthetics is hampered by the need for wires to access these bioelectric signals from the nervous system and the percutaneous connectors used to bring these signals out of the body. These extracellular potentials typically have a bandwidth of between 100 Hz to 7 KHz and have a magnitude that is less than 250 μV and may extend down to the background neural noise floor of approximately 25 μV.
As discussed above, providing transdermal connections can involve having one or more wires passing through the skin to the implanted electronic device. These wires inadvertently tether, or physically link, the devices to the skull or other fixed structures. As the soft tissues undergo normal physiological motions or volume changes devices implanted within them may move relative to other structures such as the skull connective tissue, or other bones to which the wires are intentionally affixed, or they may attach by the normal healing process. For example, the tethering of the implanted electrodes to the skull by even the finest lead wires may result in non-uniform acceleration of the tethered device due to normal head movement. This non-uniform acceleration of the implanted device can damage neurons due to the movement of the implanted device relative to the brain within the skull. This movement of the implanted device relative to the brain can lead to the damage and destruction of nearby neurons that results in a loss of signal, or a decrease in signal strength, and a loss of information. In addition, the possible long term location drift of the electrodes can result in a lack of stability of the received signals or, as described above, damage or destruction of the adjacent neurons. In addition, the electrode may move entirely out of the potential gradient in the extracellular space, thereby entirely losing the signal of interest.
RF wireless systems that have been implanted require the use of batteries that are far denser than the brain environment. In addition, the volume and mass of the batteries themselves would militate against their use in the neural environment due to compression of the surrounding neurons. RF transmission of power or data also creates substantial electrical interference for implanted devices. In addition, the use of antennas, although small due to the microwave frequencies involved are relatively large and dense relative to the local neural structures and, in addition, could heat local tissue, resulting in further damage.
Practical systems for rehabilitation of an injured spinal cord may require, for example, 1,000-100,000 electrode contacts implanted into regions of the brain associated with the control of movement. To access information from all these electrodes using wire interconnects would result in a wire bundle that would be prohibitively large and stiff, resulting in a large tethering problem. Multiplexing these thousands of signals into even a few wires would not eliminate the tethering problem. In this case the density of wires would still be so great that the differential acceleration caused by normal head motion or acceleration encountered while riding in cars, etc., may cause electrode motion resulting in damage and loss of signals. Also, as normal healing invests the wires with connective tissue, the wires may become attached to other structures resulting in other tethering locations as well.
Therefore it would be advantageous to provide a system for acquiring biomedical information and allowing for transdermal communications that does not involve tethering of the electrodes through wires for either power or signal transmission and that does not require the use of RF systems.